Regional geodynamics

Long-term reproducibility, Uncertainty, Popsat opakovana mereni

 

The repeated absolute gravity measurements are a reliable tool for monitoring geodynamic phenomena.  

To get a full benefit from these measurements, it is necessary to have, among others, a good knowledge of the instrumental accuracy and of environmental effects on gravity as the main disturbing effect.

The  data  of 8  years of AG  measurements and 2 years of SG measurements at the Pecný station allowed us to determine the accuracy parameters of the FG5#215 such as repeatability, reproducibility and uncertainty. The important parameter from the geodynamic point of view is mainly the “long-term” reproducibility and uncertainty. The “long-term” reproducibility represents the accuracy of gravity measurements with the same meter at the same site as a typical situation at a geodynamic station. The estimate   of  this  parameter  for  the  FG5#215  is  0.7 mGal (instrumental part). The uncertainty of the FG5 gravimeters of about 2.5 mGal should be considered mainly due to systematic errors between individual gravimeters. For delicate geodynamic studies, such as crustal deformations, it is useful to use only gravimeters for which the variations of systematic  errors   are  well  determined  from  regular participations in international comparison measurements of absolute gravimeters and from regular    parallel    measurements    with   a    SG   at   a reference station. It is recommended to perform repeated measurements always with the same absolute gravimeter at the given site.

Important environmental effects on gravity caused  by  the  mass  variations  in  the atmosphere and   hydrosphere   have  to  be  taken  into  account  in interpretation of repeated absolute gravity measurements. The atmospheric effects on gravity are routinely reduced with an accuracy of about 1.0 mGal, using a barometric admittance based on the local pressure information. Besides atmospheric effects, the hydrological effects cause significant disturbances in time-dependent gravity observations. The global part of the hydrological effect can be effectively removed by repeated AG measurements in the same period of the year. The local hydrological effects are therefore the main limitations for a reliable utilization of absolute gravity measurements in geodynamics. The local effects may overshadow the gravity effects associated with large scale geodynamic processes. Unfortunately they cannot be easily modelled and thus eliminated from the gravity data. The most practical way is to organize measurements always in the same time of the year to mitigate the errors coming from seasonal local hydrological effects. In that case we should calculate with an additional uncertainty of gravity measurements of 2 mGal.